1. Field of the Invention
This invention relates to an air-fuel ratio sensor deterioration-detecting device for an internal combustion engine having air-fuel ratio sensors provided in the exhaust system at respective locations upstream and downstream of a catalytic converter in the exhaust system, for detecting the air-fuel ratio of exhaust gases at respective points.
2. Prior Art
In internal combustion engines for automotive vehicles in general, the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is controlled in response to the concentration of oxygen in exhaust gases from the engine, which is detected by an air-fuel ratio sensor (hereinafter referred to as the "O2 sensor") such that the air-fuel ratio of the mixture becomes equal to a desired value.
The O2 sensor is liable to change in characteristics (internal resistance, electromotive force, and response time), due to its deterioration caused by heat and/or the like. The use of an O2 sensor which is deteriorated in characteristics, adversely affects the accuracy of the air-fuel ratio control.
To overcome this inconvenience, various proposals have been made, which include additionally providing an O2 sensor in the exhaust system at a location downstream of a catalytic converter in the exhaust system, in order to compensate for undesirable changes in characteristics of the air-fuel ratio feedback control performed in response to an output from the O2 sensor upstream of the catalytic converter, whereby the air-fuel ratio feedback control is carried out with high accuracy. According to this proposal, in controlling the air-fuel ratio of the mixture supplied to the engine to a desired value in a feedback manner responsive to the output from the upstream O2 sensor, a control amount used in the air-fuel ratio feedback control is corrected, based on an output from the downstream O2 sensor, to thereby compensate for deviation of the controlled air-fuel ratio from the desired value, which is due to deterioration of the upstream O2 sensor. However, this proposed solution has a disadvantage that when the upstream O2 sensor has become heavily deteriorated beyond the limit of the above-mentioned compensation, it will result in degraded exhaust emission characteristics of the engine.
To overcome this disadvantage, it has been proposed, e.g. by Japanese Patent Application No. 4-225284, filed Jul. 31, 1992 and its corresponding U.S. Ser. No. 08/098,049, filed Jul. 28, 1993, to measure an inversion period of an air-fuel ratio correction coefficient (air-fuel ratio correction amount) based on an output O2 sensor upstream of a catalytic converter during air-fuel ratio control using outputs from the upstream O2 sensor and an O2 sensor downstream of the catalytic converter, and determine that the upstream O2 sensor is deteriorated, when the measured inversion period exceeds a predetermined period. Then warning the user to replace the upstream O2 sensor with a fresh O2 sensor, thereby preventing the engine from being operated with degraded exhaust emission characteristics.
Further, in the above proposed control system, similarly to the conventional air-fuel ratio control system in general, predetermined upper and lower limit values are provided for the KO2 value in order to detect failure of fuel injection valves, such as fuel leakage and clogging of the valve element or the fuel supply pipe. That is, if fuel leaks from a fuel injection valve into the intake pipe, an amount of fuel supplied into the cylinder becomes larger than a desired fuel amount. As a result, the KO2 value decreases to be held at the lower limit value. On the other hand, clogging of the fuel injection valve prevents fuel from being supplied to the cylinder, so that the KO2 value increases to be held at the upper limit value.
In the above proposed air-fuel ratio control system, in addition to failure of a fuel injection valve or valves, the KO2 value decreases to be held at the lower limit value when a large amount of evaporative fuel is purged from a canister to the intake pipe.
When the KO2 value is held at the lower limit value during detection of deterioration of the catalytic converter, the actual fuel injection amount becomes larger than the desired amount, so that the output from the downstream O2 sensor continues to assume a value indicative of a rich air-fuel ratio without inversion, resulting in a longer inversion time period of the KO2 value.
On the other hand, when the KO2 value is held at the upper limit value, the actual fuel injection amount becomes smaller than the desired amount, so that the output from the downstream O2 sensor continues to assume a value indicative of a lean air-fuel ratio without inversion, also resulting in a longer inversion time period of the KO2 value.
When the KO2 value is thus held at the upper limit value or the lower limit value, the inversion period of the KO2 value becomes longer than a normal value. As a result, there is a fear that the determination of deterioration of the upstream O2 sensor cannot b accurately performed.